An HF surgical device are generally well-known, for example, from DE 10 2008 004 241 A1. FIG. 6 of this application shows a basic diagram of a known HF surgical device 1 according to DE 10 2008 004 241 A1. It comprises an HF generator 3 which produces a high-frequency output signal a, in particular a high-frequency alternating current, which is supplied to an HF surgical instrument electrically connected to the HF generator 3. The HF surgical instrument may, for example, be an instrument for argon plasma coagulation or for HF cutting which acts by means of one or a plurality of active electrodes on a patient's biological tissue 5 that is to be treated. The patient's tissue 5 to be treated acts in this case as a load impedance which is variable according to treatment level. The HF surgical device 1 further comprises a power supply unit 7 which converts the line voltage (50 Hz AC) into a voltage suitable for the HF generator.
Normally, the high frequency power is initially generated in the HF generator 3 with a carrier or fundamental frequency and is amplified in downstream amplifier stages. Depending on the application, the output signal having the fundamental frequency that is produced by the HF generator 3 is modulated over time, in particular with a variable duty cycle. To do this, it is possible either to act directly on the HF generator 3 using a modulation apparatus 9 in such a manner that the output signal is modulated by pulsing or scanning the high frequency power, i.e. by correspondingly switching the output signal on and off. This is therefore pulse-width modulation (PWM), the pulse frequency or switching frequency determining the modulation depth of the output signal. The modulation frequency fModulation results in this case from the following formula:fModulation=1/T, 
where T is the cycle duration of the square wave modulation signal. For this, the HF generator may have, for example, a circuit arrangement with MOSFET transistors which are switched on and off alternately. In this case, the modulation apparatus 9 is usually activated by a control device 11.
Another known possibility of modulating the output signal is direct activation of the power supply unit 7 by means of the control device 11. As a result of this, it becomes possible to modulate the amplitude of the output signal of the HF generator 3, the control device 11 acting as the modulation apparatus 9.
The fundamental frequency of the HF generator is normally around 300 to 500 kHz. Modulation normally takes place with frequencies between 1 kHz and 50 kHz. Preferably, however, modulation frequencies greater than 20 kHz are used to prevent the generation of unpleasant noises. However, with “pulsed argon plasma coagulation”, for example, lower modulation frequencies in the Hz range can also be used.
It is generally specified for HF surgical devices that low-frequency current fractions which may flow via the patient and consequently may trigger neuromuscular stimulation (e.g. muscle twitching) must be limited to very low values. If this specification is met, certain parts of the application may be designated as CF (cardiac floating). One drawback of these conventional low modulation frequencies is that they are below the limit above which, according to current scientific knowledge, neuromuscular phenomena, such as muscle twitching, no longer occur. It has been ascertained that this limit is approx. 100 kHz. A relevant standard, IEC 60601-2-2, therefore rules that the fundamental frequency of HF surgical devices must be greater than 200 kHz. It is overlooked here, however, that the low modulation frequencies must be taken into account even with a fundamental frequency greater than 200 kHz because they are clearly visible in the frequency spectrum of the overall output signal and may therefore induce neuromuscular phenomena.
Although it is known from the prior art to provide so-called decoupling capacitors in the patient circuit to avoid neuromuscular stimulation due to low-frequency and thus neuromuscularly effective modulation signals, said decoupling capacitors are unable to completely prevent neuromuscular stimulation which is induced in particular by the interaction of sparks between the active electrode of the surgical instrument and the tissue.